Handover (HO) refers to movement of a mobile station (MS) from a wireless interface of one base station (BS) to a wireless interface of another BS. Hereinafter, a handover procedure in a general IEEE 802.16e system will be described.
A Serving Base Station (SBS) in an IEEE 802.16e network may broadcast information about neighbor BSs through a mobile neighbor advertisement (MOB_NBR-ADV) message in order to inform an MS of information (topology) about basic network configuration.
The MOB_NBR-ADV message includes system information about a serving BS and neighbor BSs, for example, a preamble index, a frequency, a handover optimization possibility, downlink channel descriptor (DCD)/uplink channel descriptor (UCD) information, etc.
The DCD/UCD information includes information of which an MS should be aware in order to exchange information through downlink and uplink. For example, the DCD/UCD information includes handover (HO) trigger information, and medium access control (MAC) version and media independent handover (MIH) capability information of a BS.
A general MOB_NBR-ADV message includes information about neighbor BSs of an IEEE 802.16e type only. Accordingly, information about neighbor BSs of a type other than IEEE 802.16e may be broadcast to MSs through a service identity information advertisement (SII-ADV) message. As a result, an MS may acquire information about BSs of a heterogeneous network by requesting a serving BS to transmit the SII-ADV message.
A procedure for an MS having information about neighbor BSs obtained through the above-described method to perform handover in an IEEE 802.16e network will be described in more detail.
A handover (HO) procedure for use in a general IEEE 802.16e network is classified into 1) HO initiation & preparation, 2) HO execution, and 3) HO completion.
An example of the above-mentioned basic HO procedure will hereinafter be described with reference to FIG. 1.
FIG. 1 illustrates an example of a handover (HO) procedure which can be performed in an IEEE 802.16e system.
Referring to FIG. 1, an MS exchanges data with a serving BS (SBS) at step S101.
The SBS periodically broadcasts information about neighbor BSs to the MS through a MOB_NBR-ADV message at step S102.
The MS may start scanning for candidate handover (HO) BSs using a handover (HO) trigger condition while communicating with the SBS. The MS requests the SBS to perform a handover procedure by transmitting a handover request (MOB_MSHO-REQ) message when a handover condition is satisfied, for example, when a predetermined hysteresis margin value is exceeded at step S103.
The SBS informs candidate handover (HO) BSs included in the MOB_MSHO-REQ message that the MS has requested handover through a handover request (HO-REQ) message at step S104.
The candidate handover (HO) BSs, in response to the MS having requested handover, transmit information about handover to the SBS through a handover response (HO-RSP) message at step S105.
The SBS transmits the information about handover, obtained through the HO-RSP message from the candidate handover (HO) BSs, to the MS through a handover response (MOB_BSHO-RSP) message. The MOB_BSHO-RSP message may include information necessary to perform handover, that is, a handover action time, a handover identifier (HO-ID), and a dedicated handover (HO) code division multiple access (CDMA) ranging code at step S106.
The MS determines one target BS (TBS) among the candidate BSs based on the information included in the MOB-BSHO-RSP message received from the SBS. The MS then transmits a CDMA code to the determined TBS to attempt ranging at step S107.
The TBS receiving the CDMA code may inform the MS of success or failure of ranging and physical correction values through a ranging response (RNG-RSP) message at step S108.
The MS transmits a ranging request (RNG-REQ) message for authentication to the TBS at step S109.
The TBS receiving the RNG-REQ message from the MS transmits system information, which can be used in a corresponding BS, such as a connection identifier (CID) to the MS through a ranging response (RNG-RSP) message at step S110.
If the TBS successfully completes authentication of the MS and transmits all update information, the TBS informs the SBS of success or failure of handover through a handover completion (HO-CMPT) message at step S111.
It is assumed that the above-mentioned HO procedure is carried out between the MS and the BS that are based on the IEEE 802.16e standard (WirelessMAN-OFDMA Reference System). For convenience of description, a system for use in a general technology including the IEEE 802.162 standard is hereinafter referred to as a ‘legacy system’. In addition, the MS for use in the legacy technology is referred to as a ‘Yardstick MS (YMS)’ or a ‘legacy MS’, and the BS for use in the legacy technology is referred to as a ‘legacy BS’ or a Yardstick MS (YMS)’.
In addition, a specific MS, that employs the IEEE 802.16m standard (WirelessMan-OFDMA Advanced System) and an improved technology superior to a general technology, is referred to as an ‘Advanced MS (AMS)’ or a “New AMS’. A specific BS for us in the advanced technology is referred to as an ‘Advanced BS (ABS)’ or a ‘New BS’.
For convenience of description, it is assumed that AMS is connected to YBS such that it receives a necessary service from the YBS, and an ABS (WirelessMAN-OFDMA Reference System/WirelessMAN-OFDMA Advanced co-existing System) for supporting the AMS and the YMS are present in the neighborhood of the YBS.
The YBS includes only a Legacy Zone (LZone) having a physical channel frame structure applied to a legacy system. Under the condition that the ABS supports only the AMS (WirelssMAN-OFDMA advanced system only), it is assumed that the ABS includes only a new MS support zone (Mzone: 16M Zone) applied to a new system. It is assumed that the ABS (WirelessMAN-OFDMA Reference System/WirelessMAN-OFDMA Advanced co-existing Systemlegacy supportive) for supporting the AMS and the YMS includes a legacy zone (LZone) and an M zone (16M Zone). In addition, it is assumed that the ABS employs TDD (Time Division Duplex) for each of uplink and downlink in time units, for example, in units of a frame or subframe.
Moreover, it is assumed that the AMS is able to receive services from both the ABS and the YBS. That is, the AMS can receive a necessary service through either of Mzone and Lzone, and it is also assumed that the AMS is able to perform both a first HO execution process defined in the legacy system and a second HO execution process defined in a new system.
Generally, in order to allow the AMS to perform a handover (HO) from a serving YBS to the ABS supporting both the AMS and the YMS, the AMS first enters the legacy zone (LZone), such that it may continue to receive a necessary service from the LZone or may perform zone switching to the MZone (16M Zone). In addition, the AMS may immediately perform handover (HO) to the MZone without entering the LZone of the ABS.